The invention relates to a circuit for deriving a synchronizing signal contained in an incoming video signal and for applying to a line synchronizing circuit the recovered synchronizing signal, which comprises at least line synchronizing pulses whose amplitude extends between reference and peak levels comprising, connected to a video signal source a pulse generator for generating the synchronizing signal with the aid of a threshold value level generated by a threshold value detector also connected to the video signal source, this threshold value level corresponding to a level located between the peak and reference levels.
Such a circuit is disclosed in the Netherlands Patent Application No. 7000743 (PHN.4560). In this prior art circuit the threshold value level is applied to a slicer, i.e., a bidirectional limiter, for deriving the television synchronizing signal. For this purpose the peak level is determined by means of a peak level detector, whilst the reference level, which is substantially the black level of the video information of the video signal is determined with the aid of a reference level detector. The latter is a keyed detector having a switch which is operated by a suitable keying pulse. The levels thus obtained are thereafter combined for the determination of the threshold value level which is located between, for example half-way, the peak and reference levels. In the event of variations of the amplitude of the incoming synchronizing signal the relative distance of the threshold value level to the peak and reference levels remains fixed.
In the prior art circuit each of the two detectors comprises a storage element constituted by a capacitor. The invention has for its object to provide a circuit of the known type which for the determination of the threshold value level comprises only one storage element. To that end, the circuit according to the invention is characterized in that the threshold value detector is in the form of an integrator for integrating the incoming video signal during the occurrence of a first keying pulse produced during the occurrence of an incoming line synchronizing pulse and a second keying pulse produced after the occurrence of the incoming line synchronizing pulse but before the end of the line blanking interval and for storing the value obtained by this integration operation in a storage element.
If the circuit is constructed with the aid of analog components, the majority of which are included in an integrated circuit, then the circuit comprises only one capacitor so that in the region of the threshold value detector the integrated circuit needs to have only one connecting terminal.
The integration results in a signal which depends on the duration of the second keying pulse compared to the duration of the first keying pulse. If, for example, both periods of time are of equal durations, then the threshold value level obtained is located equidistantly from the peak and reference levels. The invention is based on the recognition that measurements over time are more accurate than level measurments and that they are little influenced by variations of the amplitude, also in the presence of noise, so that also the location of the level obtained varies to a small extent. An additional advantage of the measure according to the invention is that also for a small amplitude of the incoming synchronizing pulses the level obtained is located substantially in the correct position, which has for its result that a controlled amplification for obtaining an incoming signal of an adequately large amplitude can be omitted. Such an amplification is described in, for example, Netherlands Patent Specification No. 169,012.
It should be noted that the Netherlands Patent Application No. 8104533 (PHN.10,166), filed by Applicant describes a circuit similar to the present circuit comprising a peak level detector and a reference level detector which in the synchronized state of the line synchronizing circuit, that is to say in a portion of the time, operate as keyed integrators. Each integrator is supplied with a keying pulse and comprises a storage element.